Anion-decoupled electrolytes enable stable cycling and fast interfacial kinetics for calcium metal anodes

Abstract

Calcium (Ca) metal batteries are a promising post-lithium-ion technology due to the high energy density of Ca and its crustal abundance. However, the strong cation–anion interactions of divalent Ca²⁺ in conventional electrolytes promote the formation of a Ca²⁺ ion-blocking solid electrolyte interphase (SEI) that hinders Ca metal electrodeposition. Here, we introduce an anion-decoupling strategy that combines a strongly coordinating solvent, dimethylacetamide (DMAc), with an anion-pulling 1,2-dibromobenzene (1,2-DBB) additive to regulate the widely accessible Ca(TFSI)₂-based electrolytes for reversible Ca metal anodes. Spectroscopic and theoretical analyses reveal that DMAc induces a solvent-rich coordination structure and suppresses Ca²⁺-TFSI⁻ ion pairs, while 1,2-DBB further pulls TFSI⁻ out of the primary solvation shell, hindering its decomposition into unfavorable CaF₂ in the SEI. The resulting inorganic-poor SEI enables fast Ca²⁺ transport and highly reversible Ca metal plating/stripping at low overpotentials of <0.19 V, compared to those of >5 V in conventional Ca(TFSI)₂ electrolytes. The optimized electrolyte supports stable cycling of Ca//Ca symmetric cells for over 340 hours and delivers >90% capacity retention over 200 cycles in Ca//graphite and Ca//9,10-phenanthrenequinone full cells. This work establishes anion-decoupling electrolyte chemistry, distinct from the well-established anion-coupling electrolyte chemistry for monovalent metal anodes (Li and Na), enabling fast interfacial kinetics and stable cycling of divalent metal anodes.